Molecule "a" is  O2
The O2 Lewis structure has a double bond between two oxygen atoms. Â consistent with the octet rule, oxygen atoms have to bond twice. The O2 molecule is diatomic, meaning that two atoms of identical elements are connected in a pair.
What is an octet rule?
The octet rule may be a  chemical rule of thumb that reflects the theory that main-group elements tend to bond in such a way that each atom has eight electrons in its valence shell, giving it the identical electronic configuration to a noble gas.
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